1ShoppingCart would rule the eCommerce world if you just...

Enable billing to areas where shipping is restricted to.

1SC currently treats shipping and billing rescissions the same - if a country or state is elected to not be eligible for shipping, a customer cannot use a billion address in that same area (even if their shipping address is in a location that is shippable).

Creating two separate settings for acceptable Shipping jurisdictions VERSUS acceptable billing jurisdictions would be a great addition.